β2GPI complexed with HLA class II molecules is recognized by aPL antibody. (A) Possible conformations of β2GPI. Free β2GPI in serum shows a closed circular conformation, whereas β2GPI associated with phospholipids (PLs) shows a linear conformation. Because cellular misfolded proteins are presented by HLA class II molecules,³²  β2GPI with a unique conformation might also be presented by them. (B) β2GPI is displayed on the cell surface in the presence of HLA-DR. β2GPI was transfected into 293T cells together with GFP in the presence or absence of HLA-DR7 or HLA-DR8, and the transfectants were stained with anti-β2GPI, anti-HLA-DR, or aPL antibody (EY2C9) (red line). Antibody (Ab) binding to GFP-expressing cells is shown. Cells transfected with GFP alone were stained as a control (shaded histogram). (C) Direct association of β2GPI with HLA-DR. β2GPI and HLA-DR were cotransfected, and HLA-DR or β2GPI was precipitated. β2GPI and HLA-DR in the precipitates were detected by western blotting. HLA-DR or β2GPI in total cell lysates was also detected. (D) HLA-DR4 containing a covalently attached Cw4 peptide (blue lines) or wild-type HLA-DR4 (red lines) was cotransfected into 293T cells together with β2GPI and GFP. Cells transfected with β2GPI and GFP alone were used as a control (black line). β2GPI expression and aPL antibody binding to GFP-expressing cells were analyzed. Data are representative of at least 3 independent experiments.